The present invention relates to a mass flow sensor.
The mass flow sensor according to the present invention has the advantage of an improved membrane stability because of at least one adjustment layer in a membrane according to the present invention.
Due to the use of one or more preferably dielectric or nonconducting adjustment layers in a membrane according to the present invention, it is possible to design the membrane to have much greater mechanical stability, either because of a thicker membrane on the whole in comparison with the known membrane and/or because a layer sequence in the membrane according to the present invention which has a greater mechanical stability but the same layer thickness as the known membrane. Despite the greater mechanical stability with the same or greater total layer thickness of a membrane according to the present invention in comparison with the known membrane, a membrane according to the present invention has a thermal conductivity corresponding to or even exceeding that of the known membrane because of the adjustment layer(s) according to the present invention. Due to the measures according to the present invention, it is possible to produce a mass flow sensor having greater mechanical stability than the known mass flow sensor and nevertheless having a response time corresponding at least to that of the known mass flow sensor.
It is especially advantageous if at least one adjustment layer contains polycrystalline silicon.
Polycrystalline silicon, i.e. polysilicon, has a much higher thermal conductivity than silicon oxide or silicon nitride. A layer of polycrystalline silicon thus permits a more rapid dissipation of heat than a layer of silicon or silicon nitride in the same thickness. Due to the use of polycrystalline silicon or an adjustment layer of polycrystalline silicon, it is possible to increase the layer of thickness of a membrane according to the present invention in comparison with the known membrane. If a silicon oxide layer and/or a silicon nitride layer of the known membrane is replaced entirely or partially by a layer of polycrystalline silicon, a thicker membrane can be produced in comparison with the known membrane and it will have a thermal conductivity equal to or even greater than the thermal conductivity of the known membrane. Conversely, this then yields a membrane according to the present invention whose heat storage capacity is equal to or even lower than the heat storage capacity of a known membrane, so despite the greater total thickness of a membrane according to the present invention in comparison with a known membrane, it is possible to implement a mass flow sensor having at least the electric properties of the known membrane such as a rapid response time in particular.
In addition, it is advantageous if the membrane according to the present invention is formed by not only silicon oxide and silicon nitride but also other layers, all of which together are called adjustment layers. In addition to possible adjustment layers of silicon oxide and silicon nitride, these layers or adjustment layers preferably include layers of the above-mentioned polysilicon, silicon oxynitride, silicon carbide, metals or metal oxides. These metals may be, for example, platinum, titanium, palladium, nickel, aluminum, gold, chromium, tungsten or tantalum. The metal oxides may be, for example, titanium oxide, aluminum oxide, tungsten oxide or tantalum oxide. It is self-evident that these are only a few examples of implementation of the present invention.
It is advantageous that these materials for forming a membrane according to the present invention increase its mechanical stability in comparison with that of the known membrane. Furthermore, the materials according to the present invention make it possible to adjust the average thermal conductivity of a membrane according to the present invention because of the difference in their thermal conductivities and through a suitable sequence of layers or a combination of the above-mentioned adjustment layers. Preferably a membrane having greater mechanical stability than the known membrane and having an average thermal conductivity which corresponds at least to that of the known membrane is formed according to the present invention with the membrane materials mentioned above.